Aggregating fine-granular mineral salt materials



- Feb. 24, 1970 mam Em 3,497,321

AGGREGATING FINE-GRANULAR MINERAL sAm MATERIALS Filed May 10, 1966 vZ-Sheets-Sheet '1 GAP WIDTH ABOUT 0.6 T0 ABOUT l/o THE DIAMETER OF THEROLLERS.

INVENTORS HANNS DECKER, HEINZ MEDER Feb. 24, 1970 H. DECKER ET AL3,497,321

AGGREGATING FINE-GRANULAR MINERAL SALT MATERIALS 2 Sheets-Sheet 2 Filedlay 10, 1966 S m Mm 5mm V 4 wow S Z N 9 W 0' F HH n1. I

United States Patent Int. (:1. Bin 2/22 U.S. Cl. 23-252 4 ClaimsABSTRACT OF THE DISCLOSURE Method and machine for densifying andaggregating fine-granular potassium-, s0diumand similar mineral saltmaterial in a rolling mill by passing the material through the gapbetween the rollers wherein the gap has a given spacing for producing acoherent slab of the material passing between the rollers, includesadjusting the gap spacing between the rollers so that during the rollingoperation it is larger than the given spacing for producing a coherentslab and thereby producing highly densified material in the form oflongitudinal strips parallel to respective planes of roller rotationwith intermediate strips of non densified fine-granular material.

Our invention relates to a method and machinery for pressing andaggregating mineral salts, particularly potassium salts and/or sodiumsalts, in a rolling mill.

According to a known method, the more or less finegranular material ispassed through a rolling mill equipped with conventional smooth rollersbetween which the salt material is compressed and shaped into slabswhich are subjected to comminution by means of spiked rollers and thusconverted to the coarse-granular or aggregate size more advantageous forfertilizing purposes and for easier manipulation.

It has been found that, as regards the pressing operation in the rollingmill, this method may encounter difficulties on account of air beingincluded in the fine-granular salt, particularly potassium salt. The aircannot be sufficiently removed by shaking of the salt in the feed hopperor feed gap of the mill, but enters into the pressure gap between therollers of the mill together with the salt. This has caused thephenomenon that the salt slabs resulting from the conventional rollingmill operation become striated transversely to the direction of rollerrotation, the striations being formed alternately of hard zones and verysoft zones. The striation effect can be explained by the theory that theair inducted with the material will partly escape upwardly each time azone of high densification is being pressed and then loosens thematerial directly above the narrowest spot of the pressure gap. Inconsequence, the material drawn into the narrowest spot of thecompression gap during the next following compres sion phase is lesscompact and hence is pressed to a slab zone of lesser densification andstrength.

This phenomenon is the reason why the above-mentioned spike roller millshave been employed for comminution of the salt slabs, as the operationof such mills is sufiiciently delicate to prevent completelydisintegrating the zones of reduced mechanical strength and to convertthem also to coarse-granular size. The resulting product, therefore,contains a readily friable component so that its average resistance tofrictional wear is slight. Desired, however, is a higher resistance tofrictional loss because of the stresses imposed upon the granularmaterial by transportation and during distribution as a fertilizer. If

a granular product of high resistance to frictional disintegration is tobe obtained, only the zones of high strength contained in the milledslabs can be utilized in comminuted form to furnish the granularproduct, whereas the slab zones of slight mechanical strength must becompletely disintegrated by the comminuting process. This greatlyreduces the useful output delivery of the plant and increases the powerconsumption per ton of granular product since the energy employed forcompressing the zones of slight strength is wasted.

It is an object of our invention to avoid or minimize theabove-mentioned deficiencies of the known method and machinery forconverting mineral salt materials to the desired granular or aggregateconstitution.

The invention is predicated upon a surprising effect observed as soon asthe width of the pressure gap in the rolling mill exceeds a giventhreshold value. When this threshold value is exceeded, the material nolonger issues from the mill as a coherent slab extending over the entirewidth of the rolling mill but forms individual strips of highlydensified material extending longitudinally, namely in the peripheraldirection of the rollers, with non-densified fine-granular materialissuing between the densified strips. The width of the highly densifiedstrips depends upon the rotational speed of the rollers and isapproximately about 10 cm. or more. The threshold width of the gapdepends upon the size of the roller diameter and upon the constitutionof the material to be densified in the rolling mill, but in each casethe threshold width can be readily determined by a test run. As a rule,this width is approximately 0.6 to 0.7% of the roller diameter or morebut, as a rule, does not exceed about 1% of the roller diameter.

One possibility of explaining the threshold phenomenon is as follows:Due to the larger width of the gap between the rollers, the quantity ofair drawn into the gap together with the fine-granular mineral materialis so large that it can escape only toward the sides. Consequently theescaping air loosens the material on both sides of each strip of highlydensified material to such a great extent that the formation of pressedslabs in the intermediate zones is completely prevented.

According to our invention, we take advantage of this thresholdphenomenon and during operation of the rolling mill keep the width ofthe gap between the rollers at or above the threshold value so that theabove-mentioned longitudinal strips of highly densified material,extending in the direction of the roller rotation, will issue, whereasthe material between these strips is non-densified and remainsfine-granular. The strips of dense material exhibit a very highresistance to frictional wear. Hence these strip components of the milloutput can be comminuted to very strong granules with a particularlyhigh yield. The power consumption required for the pressing operationrelative to the ultimate product is comparatively slight because thereare no zones of lesser densification to be comminuted, the entirepressure energy being utilized for densifying the above-mentioned highlydensified longitudinal strips.

Preferably, the fine material passing through the roller gap between thestrips of highly densified material is separated from the densifiedmaterial by classification and may be recycled back to the rolling mill.Such separation of the fines may take place immediately following thepressing operation in the rolling mill. If desired, however, the entireoutput of the rolling mill, namely the densified strips as well as thefines, may be entered into the comminuting machinery.

It is particularly advantageous to comminute the densified strips in acrusher of the impact type, such as a hammer mill, because in suchmachinery any residual regions of slight mechanical strength, as maystill be present, are disintegrated so that the resulting granularproduct exhibits a particularly high resistance to frictional loss. Thegranular material coming from the comminuting machinery may subsequentlybe classified and the separated fines be returned to the rolling mill.

As mentioned, the minimum gap width of the rolling mill at which thehighly densified longitudinal strips with intermediate non-densifiedareas of fine material will occur, is at about 0.6 to 0.7% or more ofthe roller diameter. The upper limit of the gap width cannot bedetermined with equal definiteness because the width of the longitudinalstrips of highly densified material decreases gradually with increasingwidth of the gap. However, it has been found advisable to observe anupper limit of about 1% of the roller diameter for the narrowest widthof the gap. It is further particularly useful to employ smooth rollermills with a relatively large roller diameter of more than about 700 mm.Such rolling mills afford operating with gap widths of higher absolutevalues than mills with smaller roller diameters and can also be run athigher roller peripheral speeds thus delivering a considerably largeroutput quantity.

As mentioned, the threshold value of gap width also depends upon theconstitution of the material. With certain types of salt, the thresholdwidth is rather large so that it may not always be reliably certain thatthe densified strips have the mechanical strength required for theintended use of the granular product to be produced. It will bedesirable in such cases to provide the possibility of operating with asmaller gap width of the rolling mill, while nevertheless securing thehigh strength of the densi fied strips produced.

We have found, according to another feature of our invention, that evenwith gap widths at which a rolling mill of conventional design does notyet result in longitudinal strip formation, this phenomenon can bebrought about by providing auxiliary or accessory means which take carethat at respective localities spaced along the gap a lesserdensification of the material will occur than elsewhere in the gap.According to another feature of our invention, therefore, we provide therolling mill with a number of obstacle or impeding members in spacedrelation from each other along the gap and hence parallel to the axialdirection of the rollers, these members being located near the gap andat the material-entering side thereof so as to prevent or reduce thedensification of the material at these localities. For example, thesupply of material may be locally reduced or braked in this manner bymounting the obstacle members above the narrowest part of the rollergap. As a result, a smaller quantity of material will be drawn into thegap at the localities of the obstacle members, so that these membersdetermine from the outset respective zones in which the air, drawnelsewhere with the material into the gap, can issue out of the gap. Thelongitudinal strips remaining beside these zones then exhibit thedesired high densification and mechanical strength.

According to another, preferred feature of the invention, theabove-mentioned obstacle members are formed by elongated rods or tubeswhich protrude from above into the region of the roller gap. Whenforming these members of tubes, they are preferablysupplied withcompressed air issuing from the lower end of each tubular member. Thispermits providing a larger distance between the lower ends of the tubesand the narrowest spot of the roller gap, although in this manner alarger width of the non-densified zones will also result so that thedelivering efliciency of the pressing operation in the rolling mill isslightly reduced.

According to a further feature of our invention, the formation ofalternately highly densified and non-densified strips parallel torespective rotational planes of the rollers is secured by giving therolling-mill from the outset such a design that in predetermined zones ahigh densification of the material being pressed is prevented. This isdone, for example, by providing the peripheral surface of one or bothrollers with peripheral grooves at these localities so that at theseperipheral localities no appreciable densifying pressure is exerted uponthe material passing through the mill. If both rollers are provided withperipheral grooves, the grooves in both rollers are preferably equallyspaced and the respective grooves in one roller preferably register withthose of the other.

The invention will be further described with reference to an embodimentof a rolling mill for performing the method according to the invention,illustrated by way of example on the accompanying drawings in which:

FIG. 1 is a schematic lateral view of the rolling mill partly insection; and

FIG. 2 is a section along the line II--II in FIG. 1.

FIG. 3 is a view according to FIG. 1 showing the relative dimensions ofthe rollers and the gap, and tubes for the supply of compressed air; and

FIG. 4 is a flow diagram of the method of the invention.

The rolling mill exemplified on the drawing comprises two pressurerollers 1, 2 with smooth peripheral surfaces. The diameter of eachroller is 900 mm. for example. A hopper chute 3 for supplying thematerial to be aggregated is mounted above the rollers. A roofshapedsupport 6 is mounted between the lateral wa ls 4 of the chute 3 at alocality perpendicularly above the gap 5 formed between the two rollers.Fastened to the support 6 are a number of downwardly extending rods ortubes 7 which, relative to the axial direction of the rollers, areregularly spaced from each other, for example 140 mm. The lower ends ofthe rod-shaped members 7 terminate above the narrowest spot 8 of theroller gap 5. A sufiicient vertical spacing must remain between thelower end of each member 7 and the narrowest spot 8 of the gap. If thespacing is excessively reduced, the rods may be pulled by the enteringmaterial into the roller gap and become damaged or torn off. On theother hand, the vertical spacing of the rods 7 from the narrowest spot 8of the roller gap must not be too large because then the desired efiectcannot be obtained. The proper spacing depends largely upon the feedconditions of the particular rolling mill. In each case, however, thespacing may remain within a conveniently wide range. For example in theembodiment here being described, having a roller diameter of 900 mm., orin any event more than 700 mm., the obstacle rods may be given adiameter of A"; and, when operating the rolling mill with a narrowestgap width of 7 mm., the spacing between the bottom end of the rods 7 andthe narrowest spot 8 of the roller gap may amount to approximately mm.

As mentioned, downwardly open tubes 7 or pipes may be used instead offull-bodied rods 7. In this case, each rod is supplied from above withcompressed air to issue from the bottom opening. For this purpose thecarrier 6 is preferably designed as a horizontal manifold pipe 6' whichcommunicates with the vertical tubular rods 7' and is to be connectedwith a supply of compressed air. By thus issuing compressed air from theopenings of the tubular members, a larger spacing from the narrowestspot of the roller gap can be provided, thus improving the reliabilityof performance without forgoing the desired results.

The obstacle rods or tubes reduce the amount of material passing throughthe gap at the localities where these members are situated. It can beachieved in this manner that the formation of the densified longitudinalstrips takes place already at gap widths at which, without theseobstacle members, there would still issue a coherent slab having theabove-mentioned undesired transverse striation.

In addition to the obstacle members 7, or in lieu thereof, at least oneof the rollers 1, 2 may be provided on its peripheral surface withperipheral grooves spaced from each other in the axial direction; thislikewise,

causes the formation of zones in which the material is not, or onlynegligibly densified and through which the drawn-in air can escape fromthe adjacent strip-shaped zones as the latter are being densified in themill.

PROCESSING EXAMPLE Potassium salt having a theoretical specific gravityof 2.04 g./ccm. and a K 0 content of 40% was supplied in theconventional fine-granu ar form into a single-stage smooth rolling millhaving two rollers of 900 mm. diameter each. The mill difiered from theone shown in FIGS. 1 and 2 in that the obstacle members 6, 7 were notused. That is, the mill was a smooth roller anill of the conventionaltype. It was observed that the longitudinal strip formation commenced tooccur if the gap width was set to above 5.5 mm. Actually, the pressingoperation was performed with the gap set to 7.5 mm. at the narrowestspot. The roller peripheral speed was 1 m./sec. During operation of themill, there were produced strips of highly densified potassium saltextending i nthe rotational direction of the rollers and having a widthof about to 12 cm. individually. Between each two such strips thereremained an intermediate stripshaped zone of a few centimeter width inwhich finegranular non-densified material passed through the gap of therolling mill. The specific gravity of the stripshaped output was about1.96 kg. per liter.

According to the flow diagram of FIG. 4, the entire output delivery ofthe rolling mill was supplied to a screen-type classifier 9 in which thefines were separated from the potassium-salt strip or plate material.The fines were returned into the supply chute of the rolling mill bymeans of a pocket conveyor not shown in the flow diagram.

The potassium salt strips or pieces were subsequently comminuted in animpact mill 10 and the comminuted material then classified by a screen11 for separation of grain sizes from 0.6 to 3.5 mm. The fine fractionwas recycled back to the rolling mill. The coarse fraction, having grainsizes larger than 3.5 mm., was recycled back into the impact mill. Theoutput delivery of granules in the desired size range of 0.6 to 3.5amounted up to about 86% of the input supplied to the impact mill.Relative to this quantity of granulated material, the power requirementfor the pressing operation, inclusive of the lifting work for recyclingthe fines passing through the rolling mill without being pressed,amounted to about 10 to 10.2 kwh. per metric ton of the aggregatedproduct.

The resulting product exhibited a very high resistance to frictionalloss. Shaking tests indicated that after a shaking time of 2.5 minutesthe quantity of material removed from the granules was only about 8%.

We claim:

1. A rolling mill for producing agglomerated potassium, sodium andsimilar mineral salt material, comprising two opposingly rotatingrollers forming a gap between each other, means for supplyingfine-granular salt material to the gap, and obstacle members mounted atsaid gap on the supply side of said rollers, said obstacle membershaving free end portions, respectively, extending into said gap butterminating short of the narrowest part of said gap, said members beingfixedly spaced from each other at respective localities along said gapfor minimizing the densification of the material at said localities,said gap having a width of about 0.6 to about 1% the diameter of saidrollers so as to produce highly densified material in the form oflongitudinal strips spaced from one another over the width of saidrollers with substantially non-densified fine-granular material disposedtherebetween at said localities.

2. In a rolling mill according to claim 1, said supply means forming achute above said gap, and said members being mounted in the bottomportion of said chute above the narrowest part of said gap.

3. In a rolling mill according to claim 2, said members being rod-shapedand downwardly elongated into the region of said gap.

4. In a rolling mill according to claim 3, said rodshaped members beingtubular, open at the lower end, and forming part of means for issuingair under pressure from said open ends.

References Cited UNITED STATES PATENTS 1,459,082 6/1923 Bartlett 232932,922,189 1/1960 Perks 173 3,145,418 8/1964 Kusters 18-9 3,223,02612/1965 Flemming 10090 3,282,199 11/1966 Mason 18-9 3,029,723 4/ 1962Schweer 100-90 NORMAN YUDKOFF, Primary Examiner US. Cl. X.R.

